Composite ceramic structures

ABSTRACT

A self-supporting ceramic composite body produced by a method which comprises preparing a polycrystalline material as the oxidation reaction product of a parent metal with a vapor-phase oxidant, comminuting the resulting material to a particulate, forming a permeable mass of said particulate as filler, and infiltrating said particulate with an oxidation reaction product of a parent metal with a vapor-phase oxidant, thereby forming said ceramic composite body.

This is a continuation of copending application Ser. No. 541,437 filedJun. 22, 1990 which issued on Oct. 1, 1991, as U.S. Pat. No. 5,053,367,which was a Rule 62 continuation of application Ser. No. 267,450, filedon Nov. 4, 1988, now abandoned, which was a Rule 60 continuation ofapplication Ser. No. 052,806, filed on May 21, 1987, and which issued toU.S. Pat. No. 4,833,110 on May 23, 1989. Application Ser. No. 052,806was a continuation-in-part of application Ser. No. 908,123, filed onSep. 16, 1986, and now abandoned.

FIELD OF THE INVENTION

This invention relates to a novel method for producing a ceramiccomposition body. More particularly, this invention relates to animprovement in the method for producing a ceramic composite body by"growing" a polycrystalline material comprising an oxidation reactionproduct from a parent metal into a permeable mass of filler comprisingcomminuted particles of a polycrystalline material produced anteriorlyby essentially the same generic process.

BACKGROUND

In recent years there has been an increasing interest in substitutingceramics for metals because, with respect to certain properties,ceramics are superior to metals. There are, however, several knownlimitations or difficulties in making this substitution such as scalingversatility, capability to produce complex shapes, satisfying theproperties required for the end-use application, and costs. Many ofthese limitations or difficulties have been overcome by the inventionsdisclosed in patents and patent applications assigned to the sameassignee as this application and discussed in the subsequent section,which provide novel methods for reliably producing ceramic materials,including shaped composites.

DESCRIPTION OF COMMONLY OWNED PATENTS AND PATENT APPLICATIONS

The following commonly owned patents and patent applications describenovel methods for producing a self-supporting ceramic body by oxidationof a parent to form a polycrystalline oxidation reaction product and,optionally, metallic constituents:

(A) U.S. Pat. No. 4,713,360, which issued on Dec. 15, 1987, from U.S.patent application Ser. No. 818,943, filed Jan. 15, 1986, which is acontinuation-in-part of Ser. No. 776,964, filed Sep. 17, 1985, and nowabandoned, which was a continuation-in-part of Ser. No. 705,787, filedFeb. 26, 1985, and now abandoned, which was a continuation-in-part ofU.S. application Ser. No. 591,392, filed Mar. 16, 1984, and nowabandoned, all in the names of Marc S. Newkirk et al and entitled "NovelCeramic Materials and Methods for Making the Same";

(B) U.S. Pat. No. 4,853,352, which issued on Aug. 1, 1989, from U.S.patent application Ser. No. 220,935, filed on Jun. 23, 1988, which is acontinuation of U.S. patent application Ser. No. 822,999, filed Jan. 27,1986, and now abandoned, which was a continuation-in-part of Ser. No.776,965, filed Sep. 17, 1985, and now abandoned, which was acontinuation-in-part of Ser. No. 747,788, filed Jun. 25, 1985, and nowabandoned, which was a continuation-in-part of Ser. No. 632,636, filedJul. 20, 1984, and now abandoned, all in the names of Marc S. Newkirk etal and entitled "Methods of Making Self-Supporting Ceramic Materials";and

(C) U.S. Pat. No. 4,851,375, which issued on Jul. 25, 1989, from U.S.patent application Ser. No. 819,397, filed Jan. 17, 1986, which is acontinuation-in-part of Ser. No. 697,876, filed Feb. 4, 1985, and nowabandoned, both in the names of Marc S. Newkirk et al and entitled"Composite Ceramic Articles and Methods of Making Same".

The entire disclosures of each of the aforesaid Commonly Owned Patentsand Patent Applications are incorporated herein by reference.

As explained in these Commonly Owned Patents and Patent Applications,novel polycrystalline ceramic materials or polycrystalline ceramiccomposite materials are produced by the oxidation reaction between aparent metal and a vapor-phase oxidant, i.e. a vaporized or normallygaseous material, as an oxidizing atmosphere. The method is disclosedgenerically in the aforesaid Commonly Owned Patent Applications (A) U.S.Pat. No. 4,713,360. In accordance with this generic process, a parentmetal, e.g. aluminum, is heated to an elevated temperature above itsmelting point but below the melting point of the oxidation reactionproduct to form a body of molten parent metal which reacts upon contactwith a vapor-phase oxidant to form the oxidation reaction product. Atthis temperature, the oxidation reaction product, or at least a portionthereof, is in contact with and extends between the body of moltenparent metal and the oxidant, and molten metal is drawn or transportedthrough the formed oxidation reaction product and towards the oxidant.The transported molten metal forms additional oxidation reaction productupon contact with the oxidant, at the surface of previously formedoxidation reaction product. As the process continues, additional metalis transported through this formation of polycrystalline oxidationreaction product thereby continually "growing" a ceramic structure ofinterconnected crystallites. The resulting ceramic body may containmetallic constituents, such as non-oxidized constituents of the parentmetal, and/or voids. In the case of an oxide as the oxidation reactionproduct, oxygen or gas mixtures containing oxygen (including air) aresuitable oxidants, with air usually being preferred for obvious reasonsof economy. However, oxidation is used in its broad sense in all of theCommonly Owned Patents and Patent Applications and in this application,and refers to the loss or sharing of electrons by a metal to an oxidantwhich may be one or more elements and/or compounds. Accordingly,elements other than oxygen, or compounds, may serve as the oxidant, asexplained below in greater detail.

In certain cases, the parent metal may require the presence of one ormore dopants in order to favorably influence or facilitate growth of theoxidation reaction product, and the dopants are provided as alloyingconstituents of the parent metal. For example, in the case of aluminumas the parent metal and air as the oxidant, dopants such as magnesiumand silicon, to name but two of a larger class of dopant materials, arealloyed with aluminum and utilized as the parent metal. The resultingoxidation reaction product comprises alumina, typically α-alumina.

The aforesaid Commonly Owned Patent Applications (B) U.S. Pat. No.4,853,352 discloses a further development based on the discovery thatappropriate growth conditions as described above, for parent metalsrequiring dopants, can be induced by applying one or more dopantmaterials to the surface or surfaces of the parent metal, thus avoidingthe necessity of alloying the parent metal with dopant materials, e.g.metals such as magnesium, zinc and silicon, in the case where aluminumis the parent metal and air is the oxidant. With this improvement, it isfeasible to use commercially available metals and alloys which otherwisewould not contain or have appropriately doped compositions. Thisdiscovery is advantageous also in that ceramic growth can be achieved inone or more selected areas of the parent metal's surface rather thanindiscriminately, thereby making the process more efficiently applied,for example, by doping only one surface, or only portion(s) of asurface, of a parent metal.

Novel ceramic composite structures and methods of making them aredisclosed and claimed in the aforesaid Commonly Owned PatentApplications (C) U.S. Pat. No. 4,851,375 which utilizes the oxidationreaction to produce ceramic composite structures comprising asubstantially inert filler infiltrated by the polycrystalline ceramicmatrix. A parent metal positioned adjacent to a mass of permeable filleris heated to form a body of molten parent metal which is reacted with avapor-phase oxidant, as described above, to form an oxidation reactionproduct. As the oxidation reaction product grows and infiltrates theadjacent filler material, molten parent metal is drawn throughpreviously formed oxidation reaction product into the mass of filler andreacts with the oxidant to form additional oxidation reaction product atthe surface of the previously formed product, as described above. Theresulting growth of oxidation reaction product infiltrates or embeds thefiller and results in the formation of a ceramic composite structure ofa polycrystalline ceramic matrix embedding the filler.

Thus, the aforesaid Commonly Owned Patents and Patent Applicationsdescribe the production of oxidation reaction products readily "grown"to desired sizes and thicknesses heretofore believed to be difficult, ifnot impossible, to achieve with conventional ceramic processingtechniques. The present invention provides a further improvement for usein the production of ceramic composite products.

SUMMARY OF THE INVENTION

This invention relates to an improved method for producing apolycrystalline ceramic composite body by infiltrating a permeable massor bed of filler with a ceramic matrix comprising a polycrystallineoxidation reaction product grown by the oxidation of a molten parentmetal in accordance with the aforesaid Commonly Owned Patents and PatentApplications. The filler comprises a comminuted version ofpolycrystalline material also made in accordance with the aforesaidCommonly Owned Patents and Patent Applications. Using a filler which isa substantial replicate (but not necessarily an exact replicate) of theceramic material made anteriorly to the composite product by essentiallythe same process provides for enhanced kinetics and improved morphology,as described below in greater detail.

In the practice of this invention, a parent metal is heated in thepresence of a vapor-phase oxidant to form a body of molten metal whichis in contact with a bed of permeable filler. Oxidation reaction productis formed as molten metal contacts the oxidant, and the processconditions are maintained to progressively draw molten metal through theformed oxidation reaction product and toward the oxidant so as tocontinuously form oxidation reaction product at the interface betweenthe oxidant and previously formed oxidation reaction product.

The heating step is conducted at temperatures above the melting point ofthe parent metal but below the melting temperature of the oxidationreaction product and heating is continued for such time as is necessaryto produce a polycrystalline ceramic body of the desired size. The bodymay include one or more metallic constituents such as nonoxidized parentmetal, or voids, or both.

The improvement of this invention is based on the discovery that aself-supporting ceramic composite body can be obtained by utilizing as afiller a comminuted replicate or form of the polycrystalline materialobtained according to the oxidation reaction process described in thisSummary section and in greater detail in the Commonly Owned PatentApplications. The polycrystalline material thus obtained as afirst-stage is ground, pulverized, or the like, and a mass of theresulting filler, preferably shaped as a permeable preform, is placedadjacent to a second body of the parent metal and the resulting assemblyis subjected to the oxidation reaction process as a second-stage. Thisreaction process is continued for a time sufficient to infiltrate atleast a portion of the filler bed with the polycrystalline oxidationreaction product formed from the second parent metal so that a ceramiccomposite body of the desired dimensions can be obtained.

More specifically, a second parent metal is positioned or orientedrelative to the permeable mass of filler material so that formation ofthe oxidation reaction product from the second parent metal will occurin a direction towards and into the mass of filler. The growth ofoxidation reaction product infiltrates or embeds the mass of fillerthereby forming the desired composite ceramic structure. The filler maybe a loose or bonded array characterized by interstices, openings orintervening spaces, and the bed or mass is permeable to the vapor-phaseoxidant and to the growth of oxidation reaction product. As used hereinand in the appended claims, "filler" or "filler material" is intended tomean either a homogeneous composition or a heterogeneous compositioncomprised of two or more materials. Thus, the filler may have admixedwith it one or more additional filler materials which may be prepared byconventional methods. Still further, the parent metals and the oxidantsused in the process for providing the replicated filler may besubstantially the same or different in composition from that used inproducing the final composite product.

The oxidation reaction product grows into the filler without disruptionor displacement of the filler constituents as a result of which arelatively dense composite ceramic body is formed without the use ofhigh temperatures and high pressures. Moreover, the present processreduces or obviates the need for chemical and physical compatability,conditions which are generally required when pressureless sinteringtechniques are employed in ceramic composite production.

The ceramic composite bodies which are produced by the present inventionexhibit highly desirable electrical, wear, thermal and structuralcharacteristics and, if necessary, they may be machined, polished,ground, or the like to afford products which have a variety ofindustrial applications.

As used in this specification and the appended claims, the followingterms have the following meaning:

"Ceramic" is not to be unduly construed as being limited to a ceramicbody in the classical sense, that is, in the sense that it consistsentirely of non-metallic and inorganic materials, but rather refers to abody which is predominantly ceramic with respect to either compositionor dominant properties, although the body may contain minor orsubstantial amounts of one or more metallic constituents derived fromthe parent metal, or reduced from the oxidant or a dopant, mosttypically within the range of from about 1-40% by volume, but mayinclude still more metal.

"Oxidation reaction product" means one or more metals in any oxidizedstate wherein the metal(s) have given up electrons to or sharedelectrons with another element, compound, or combination thereof.Accordingly, an "oxidation reaction product" under this definitionincludes the product of the reaction of one or more metals with anoxidant such as oxygen, nitrogen, a halogen, sulphur, phosphorus,arsenic, carbon, boron, selenium, tellurium, and compounds andcombinations thereof including, for example, methane, oxygen, ethane,propane, acetylene, ethylene, propylene, and mixtures such as air, H₂/H₂ O and a CO/CO₂, the latter two (i.e., H₂ /H₂ O and CO/CO₂) beinguseful in reducing the oxygen activity of the environment.

"Oxidant", "vapor-phase oxidant" or the like, which identifies theoxidant as containing or comprising a particular gas or vapor, means anoxidant in which the identified gas or vapor is the sole, orpredominant, or at least a significant oxidizer of the parent metalunder the conditions obtained in the oxidizing environment utilized. Forexample, although the major constituent of air is nitrogen, the oxygencontent of air is the sole oxidizer for the parent metal because oxygenis a significantly stronger oxidant than nitrogen. Air therefore fallswithin the definition of an "oxygen-containing gas" oxidant but notwithin the definition of a "nitrogen-containing gas" oxidant as thoseterms are used herein and in the claims. An example of a"nitrogen-containing gas" oxidant as used herein and in the claims is"forming gas", which typically contains about 96 volume percent nitrogenand about 4 volume percent hydrogen.

"Parent metal" refers to that metal, e.g. aluminum, which is theprecursor for the polycrystalline oxidation reaction product, andincludes that metal as a relatively pure metal, a commercially availablemetal with impurities and/or alloying constituents, or an alloy in whichthat metal precursor is the major constituent; and when a specifiedmetal is mentioned as the parent metal, e.g. aluminum, the metalidentified should be read with this definition in mind unless indicatedotherwise by the context.

BRIEF DESCRIPTION OF THE FIGURES

FIGS. 1A and 1B are photographs showing the external growth morphologyof specimens obtained by oxidizing a parent metal of aluminum alloy380.1 into particle beds of, in the case of FIG. 1A, a grown and crushedalumina material and, in the case of FIG. 1B, a fused alumina material.

FIGS. 2A and 2B are photographs showing the external growth morphologyof specimens obtained by oxidizing a parent metal of 99.7% pure aluminuminto particle beds of, in the case of FIG. 2A, a grown and crushedalumina material and, in the case of FIG. 2B, a fused alumina material.

DETAILED DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS

In accordance with the present invention for producing self-supportingceramic composite bodies, a parent metal is heated to a molten state inthe presence of a vapor-phase oxidant to form an oxidation reactionproduct which infiltrates a bed or mass of filler. The filler utilizedis comminuted particles of the polycrystalline material producedanteriorly by substantially the same process (i.e., as a first-state).This filler exhibits an affinity for the oxidation reaction productgrown during the process for producing the final composite product,(i.e., in a second-stage) apparently attributable to an affinity betweenlike substances under the process conditions; that is, there is anapparent affinity for growing reaction product into its replicate.Because of this affinity, we have observed enhanced growth kinetics, andconsequently growth occurs at a somewhat faster rate relative tosubstantially the same process not using a replicated filler. Inaddition, we have observed an improvement in morphology, contributing tothe high quality replication of a parent metal pattern by the ceramicbody, as fully illustrated in the examples below, and disclosed incopending U.S. Pat. No. 4,828,785, which issued on May 9, 1989, fromU.S. patent application Ser. No. 823,542, filed on Jan. 27, 1986, andassigned to the same assignee, and incorporated herein by reference.

One factor which appears to contribute to these improved characteristicsis the presence of a dopant material intimately associated with thefiller. For example, when alumina as the oxidation reaction product isformed upon the oxidation reaction of aluminum in air, a dopant materialtypically is used in association with or in combination with thealuminum parent metal. The dopant, or a portion thereof, may not beexhausted from the reaction system, and therefore becomes dispersedthrough part or all of the polycrystalline material. In such a case, thedopant material may be concentrated in the initiation surface of theexternal surface of the polycrystalline material, or may be intimatelybonded with the microstructure of the oxidation reaction product, or maybe alloyed with the metallic component of the polycrystalline material.When the polycrystalline material is comminuted for use as a filler,this dopant material incorporated as part of the filler now serves as auseful dopant in the production of the final composite product. Forexample, silicon is a useful dopant for the oxidation reaction ofaluminum in air, and a significant percentage of silicon will alloy withthe metal phase of the polycrystalline material. When used as a filler,this polycrystalline material contains a built-in dopant for use inmaking an alumina composite.

The ceramic body produced as the source of filler for the finalcomposite product is comminuted to the desired size as by impactmilling, roller milling, gyratory crushing, or other conventionaltechniques depending largely upon the particle size desired and thecomposition of the polycrystalline material. The ground or milledceramic material is sized and recovered for use as a filler. It may bedesirable to first crush the ceramic body into large pieces of about 1/4inch to 1/2 inch as with a jaw crusher, hammer mill, etc., and then intofiner particle of 50 mesh or finer as by impact milling. The particulateis typically screened to obtain fractions of desired size. Suitablefillers may range in size from 100 to 500 mesh or finer depending uponthe ceramic composite body to be made and its end use.

As explained above, the polycrystalline material formed may containmetallic components such as nonoxidized parent metal. The amount ofmetal can vary over a wide range of 1 to 40 percent by volume, andsometimes higher, depending largely upon the degree of exhaustion(conversion) of parent metal used in the process. It may be desirable toseparate at least some of the metal, particularly the larger sections,from the oxidation reaction product before using the material as afiller. This separation can be conveniently achieved after thepolycrystalline material has been crushed or ground. The oxidationreaction product is usually more easily fractured than the metal, and ittherefore may be possible in some cases to partially separate the twoconstituents by comminuting and screening.

Also, any unoxidized parent metal present in the filler will be inparticulate form, and when used in forming a final product, will undergooxidation reaction leaving voids in the ceramic matrix corresponding insize to the metal particles. Such voids disposed throughout the ceramicmatrix may or may not be desirable depending upon the properties soughtfor the composite and its end use. If a high volume percent of voids isdesirable for the end product, such as for increasing the thermalinsulation of the composite, it would be advantageous to use fillerhaving a substantial amount of nonoxidized parent metal. This built-inporosity can be restricted to a portion of the composite only by forminga layered bed of filler comprising (1) filler with particulate parentmetal and (2) the relatively pure filler (metal removed) or filler fromanother source.

It will be observed that in accordance with the present invention, theparent metal used in producing the filler may be substantially the sameor different from the parent metal used in producing the final ceramiccomposite product. This may be desirable in that it affords the use of afiller having the several advantages enumerated above, but the oxidationreaction product is different in chemical composition from the oxidationreaction product of the final product. For example, by this embodimentit is possible to form an aluminum oxide ceramic body by the oxidationreaction process of an aluminum parent metal in an oxygen atmosphere forsubsequent use as a filler in a ceramic matrix of aluminum-nitrideformed upon the oxidation reaction of an aluminum parent metal in anitrogen atmosphere.

In an alternative embodiment, the filler utilized in producing the finalcomposite product is itself derived from a ceramic composite formed bythe oxidation reaction process, and then comminuted and sieved to size.The filler used in making the ceramic composite body which is theprecursor filler to the final product, may be selected to augment orimprove the properties of the final product. This may be achieved byselecting a filler different in composition from the oxidation reactionproduct so that the resulting precursor filler will be composed of orcontain two components which may be intimately bonded as amicrocomposite. In making the ceramic composite structure body by thisembodiment, a first source of parent metal and a permeable bed or massof filler material are oriented relative to each other so that formationof the oxidation reaction product will occur in a direction towards andinto said bed of filler material. The first source of parent metal isheated in the presence of a vapor-phase oxidant to form a body of moltenparent metal which reacts with the oxidant in this temperature region toform an oxidation reaction product. The oxidation reaction product is incontact with, and extends between, the body of molten metal and theoxidant, thereby progressively drawing molten metal through theoxidation reaction product towards the oxidant and into the mass offiller material so that the oxidation reaction product continues to format the interface between the oxidant and previously formed oxidationreaction product. The reaction is continued for a time sufficient toinfiltrate at least a portion of the bed of filler material with apolycrystalline material comprising oxidation reaction product and,optionally, one or more metallic constituents such as nonoxidized parentmetal. The resulting polycrystalline composite body is comminuted to aparticulate size suitable for use as a second filler, and a permeablemass of this second filler (which may be of a different composition) isoriented relative to a second source of parent metal so that formationof an oxidation reaction product will occur in a direction towards andinto this mass of second filler. The oxidation reaction process isrepeated as set forth above, and continued for a time sufficient so thatthe oxidation reaction product infiltrates at least a portion of themass of second filler thereby forming the final ceramic compositeproduct.

The properties of the ceramic composite products of this invention canvary depending upon such factors as the choice of parent metal, thecomposition of the fillers, and the oxidant. Typical properties soughtfor these composites, which can be tailored, include hardness, flexuralstrength, fracture toughness and elastic modulus. The composite productsgenerally are adaptable or fabricated, such as by machining, polishing,grinding, etc., for use as articles of commerce which, as used herein,are intended to include, without limitation, industrial, structural, andtechnical ceramic bodies for applications where electrical, wear,thermal, structural or other features or properties are important orbeneficial.

Although the present invention is described herein with particularemphasis on systems wherein aluminum or an aluminum alloy is employed asthe parent metal and alumina is the intended oxidation reaction product,this reference is for exemplary purposes only, and it is to beunderstood that the parent invention is adaptable by application of theteachings herein to other systems wherein other metals such as tin,silicon, titanium, zirconium, etc., are employed as the parent metal.Further, the intended oxidation reaction product is metal oxide,nitride, boride, carbide, and the like, of the parent metal.

In somewhat greater detail as to certain process steps, the parent metal(which may be doped, as explained above) as the precursor to theoxidation reaction product, is formed into an ingot, billet, rod, plate,or the like, and placed in an inert bed, crucible or other refractorycontainer. This container with its contents is placed in a furnace whichis supplied with a gas oxidant. This setup is heated to temperaturesbelow the melting point of the oxidation reaction product but above themelting point of the parent metal which temperature region, for example,in the case of aluminum using air as the vapor-phase oxidant, isgenerally between about 850°-1450° C. and more preferably between about900°-1350° C. Within this operable temperature interval or range, a bodyor pool of molten metal forms, and on contact with the oxidant, themolten metal will react to form a layer of oxidation reaction product.Upon continued exposure to the oxidizing environment, molten metal isprogressively drawn into and through any previously formed oxidationreaction product in the direction of the oxidant. On contact with theoxidant, the molten metal will react to form additional oxidationreaction product and thus form a progressively thicker oxidationreaction product while, optionally, leaving metallic constituentsdispersed through the polycrystalline material. The reaction of themolten metal with the oxidant is continued until the oxidation reactionproduct has grown to a desired limit or boundary.

In the embodiment where a ceramic composite body is prepared to serve asthe precursor filler, the parent metal and a permeable mass of fillermaterial are positioned adjacent to each other and oriented with respectto each other so that growth of the oxidation reaction product asdescribed above will be in a direction towards the filler material inorder that the filler, or a part thereof, will be infiltrated by thegrowing oxidation reaction product and embedded therein. Thispositioning and orientation of the parent metal and filler with respectto each other may be accomplished by simply embedding a body of parentmetal within a bed of particulate filler material or by positioning oneor more bodies of parent metal within, or or adjacent to a bed or otherassembly of filler material. The assembly is arranged so that adirection of growth of the oxidation reaction product will permeate orinfiltrate at least a portion of the filler material. The filler maycomprise, for example, powders or other particulates, aggregate,refractory fiber, tubules, whiskers, spheres, platelets, or the like ora combination of the foregoing. Further, suitable filler materials mayinclude, for example, metal oxides, nitrides or carbides such asalumina, magnesia, hafnia, zirconia, silicon carbide, silicon nitride,zirconium nitride, titanium nitride, etc., as explained in the CommonlyOwned Patents and Patent Applications.

The resulting polycrystalline material may exhibit porosity which may bea partial or nearly complete replacement of the metal phase(s), but thevolume percent of voids will depend largely on such conditions astemperature, time, type of parent metal, and dopant concentrations.Typically in these polycrystalline ceramic structures, the oxidationreaction product crystallites are interconnected in more than onedimension preferably in three dimensions, and the metal may be at leastpartially interconnected.

The polycrystalline ceramic material (or composite material if made) isnow comminuted and sized for use as a filler in producing the finalcomposite product. This particulate filler, which may be admixed withstill other filler materials, is formed into a permeable bed, preferablyinto a shaped preform. The bed and a second parent metal are orientedrelative to each other so that formation of an oxidation reactionproduct will occur in a direction towards and into the bed. The processsteps as outlined above are essentially repeated. The reaction processis continued for a time sufficient so that oxidation reaction productinfiltrates at least a portion of the bed, or to the desired boundary ofthe preform, thereby forming the ceramic composite body.

A particularly effect method for practicing this invention involvesforming the filler into a preform with a shape corresponding to thedesired geometry of the final composite product. The preform may beprepared by any of a wide range of conventional ceramic body formationmethods (such as uniaxial pressing, isostatic pressing, slip casting,sedimentation casting, tape casting, injection molding, filament windingfor fibrous materials, etc.) depending largely on the characteristics ofthe filler. Initial binding of the particles prior to infiltration maybe obtained through light sintering or by use of various organic orinorganic binder materials which do not interfere with the process orcontribute undesirable by-products to the finished material. The preformis manufactured to have sufficient shape integrity and green strength,and is thus self-supporting. The preform should also be permeable to thetransport of oxidation reaction product, preferably having a porosity ofbetween about 5 and 90% by volume and more preferably between about 25and 50% by volume. Also, an admixture of filler materials and mesh sizesmay be used. The preform is then contacted with molten parent metal onone or more of its surfaces for a time sufficient to complete growth andinfiltration of the preform to its surface boundaries.

As disclosed in copending and allowed U.S. patent application Ser. No.861,024, filed on May 8, 1989, and assigned to the same owner, a barriermeans may be used in conjunction with the filler material or preform toinhibit growth or development of the oxidation reaction product beyondthe barrier. Suitable barrier means may be any material, compound,element, composition, or the like, which, under the process conditionsof this invention, maintains some integrity, is not volatile, andpreferably is permeable to the vapor-phase oxidant while being capableof locally inhibiting, poisoning, stopping, interfering with,preventing, or the like, continued growth of oxidation reaction product.Suitable barriers for use with aluminum parent metal include calciumsulfate (Plaster of Paris), calcium silicate, and Portland cement, andmixtures thereof, which typically are applied as a slurry or paste tothe surface of the filler material. These barrier means also may includea suitable combustible or volatile material that is eliminated onheating, or a material which decomposes on heating, in order to increasethe porosity and permeability of the barrier means. Still further, thebarrier means may include a suitable refractory particulate to reduceany possible shrinkage or cracking which otherwise may occur during theprocess. Such a particulate having substantially the same coefficient ofexpansion as that of the filler bed or preform is especially desirable.For example, if the preform comprises alumina and the resulting ceramiccomprises alumina, the barrier may be admixed with alumina particulate,desirably having a mesh size of about 20-1000, but may be still finer.Other suitable barriers include refractory ceramics or metal sheathswhich are open on at least one end to permit a vapor-phase oxidant topermeate the bed and contact the molten parent metal.

As a result of using a preform, especially in combination with a barriermeans, a net shape is achieved, thus minimizing or eliminating expensivefinal machining or grinding operations.

As a further embodiment of the invention and as explained in theCommonly Owned Patents and Patent Applications, the addition of dopantmaterials in conjunction with the parent metal can favorably influencethe oxidation reaction process. The function or functions of the dopantmaterial can depend upon a number of factors other than the dopantmaterial itself. These factors include, for example, the particularparent metal, the end product desired, the particular combination ofdopants when two or more dopants are used, the use of an externallyapplied dopant in combination with an alloyed dopant, the concentrationof the dopant, the oxidizing environment, and the process conditions.

The dopant or dopants used in conjunction with the parent metal (1) maybe provided as alloying constituents of the parent metal, (2) may beapplied to at least a portion of the surface of the parent metal, or (3)may be applied to the filler bed or preform or to a part thereof, e.g.,the support zone of the preform, or any combination of two or more oftechniques (1), (2), and (3) may be employed. For example, an alloyeddopant may be used in combination with an externally applied dopant. Inthe case of technique (3), where a dopant or dopants are applied to thefiller bed or preform, the application may be accomplished in anysuitable manner, such as by dispersing the dopants throughout part orthe entire mass of the preform as coatings or in particulate form,preferably including at least a portion of the preform adjacent theparent metal. Application of any of the dopants to the preform may alsobe accomplished by applying a layer of one or more dopant materials toand within the preform, including any of its internal openings,interstices, passageways, intervening spaces, or the like, that renderit permeable. A convenient manner of applying any of the dopant materialis to merely soak the entire bed in a liquid (e.g., a solution) ofdopant material. As explained above, the dopant may be built into thefiller which is used in producing the final composite product. A sourceof the dopant may also be provided by placing a rigid body of dopant incontact with and between at least a portion of the parent metal surfaceand the preform. For example, a thin sheet of silicon-containing glass(useful as a dopant for the oxidation of an aluminum parent metal) canbe placed upon a surface of the parent metal. When the aluminum parentmetal (which may be internally doped with Mg) overlaid with thesilicon-containing material is melted in an oxidizing environment (e.g.,in the case of aluminum in air, between about 850° C. to about 1450° C.,preferably about 900° C. to about 1350° C.), growth of thepolycrystalline ceramic material into the permeable preform occurs. Inthe case where the dopant is externally applied to at least a portion ofthe surface of the parent metal, the polycrystalline oxide structuregenerally grows within the permeable preform substantially beyond thedopant layer (i.e., to beyond the depth of the applied dopant layer). Inany case, one or more of the dopants may be externally applied to theparent metal surface and/or to the permeable preform. Additionally,dopants alloyed within the parent metal and/or externally applied to theparent metal may be augmented by dopant(s) applied to the preform. Thus,any concentration deficiencies of the dopants alloyed within the parentmetal and/or externally applied to the parent metal may be augmented byadditional concentration of the respective dopant(s) applied to thepreform and vice versa.

Useful dopants for an aluminum parent metal, particularly with air asthe oxidant, include, for example, magnesium, zinc, and silicon, incombination with each other or in combination with other dopantsdescribed below. These metals, or a suitable source of the metals, maybe alloyed into the aluminum-based parent metal at concentrations foreach of between about 0.1-10% by weight based on the total weight of theresulting doped metal. Concentrations within this range appear toinitiate the ceramic growth, enhance metal transport and favorablyinfluence the growth morphology of the resulting oxidation reactionproduct. The concentration range for any one dopant will depend on suchfactors as the combination of dopants and the process temperature.

Other dopants which are effective in promoting polycrystalline oxidationreaction product growth, for aluminum-based parent metal systems are,for example, germanium, tin and lead, especially when used incombination with magnesium or zinc. One or more of these other dopants,or a suitable source of them, is alloyed into the aluminum parent metalsystem at concentrations for each of from about 0.5 to about 15% byweight of the total alloy; however, more desirable growth kinetics andgrowth morphology are obtained with dopant concentrations in the rangeof from about 1-10% by weight of the total parent metal alloy. Lead as adopant is generally alloyed into the aluminum-based parent metal at atemperature of at least 1000° C. so as to make allowances for its lowsolubility in aluminum; however, the addition of other alloyingcomponents, such as tin, will generally increase the solubility of leadand allow the alloying materials to be added at a lower temperature.

One or more dopants may be used depending upon the circumstances, asexplained above. For example, in the case of an aluminum parent metaland with air as the oxidant, particularly useful combinations of dopantsinclude (a) magnesium and silicon or (b) magnesium, zinc and silicon. Insuch examples, a preferred magnesium concentration falls within therange of from about 0.1 to about 3% by weight, for zinc in the range offrom about 1 to about 6% by weight, and for silicon in the range of fromabout 1 to about 10% by weight.

Additional examples of dopant materials, useful with an aluminum parentmetal, include sodium, lithium, calcium, boron, phosphorus and yttrium,which may be used individually or in combination with one or more otherdopants depending on the oxidant and process conditions. Sodium andlithium may be be used in very small amounts in the parts per millionrange, typically about 100-200 parts per million, and each may be usedalone or together, or in combination with other dopant(s). Rare earthelements such as cerium, lanthanum, praseodymium, neodymium and samariumare also useful dopants, and herein again especially when used incombination with other dopants.

As noted above, it is not necessary to alloy any dopant material intothe parent metal. For example, selectively applying one or more dopantmaterials in a thin layer to either all, or a portion of, the surface ofthe parent metal enables local ceramic growth from the parent metalsurface or portions thereof and lends itself to growth of thepolycrystalline ceramic material into a permeable bed or preform inselected areas. Thus, growth of the polycrystalline ceramic material canbe controlled by the localized placement of the dopant material upon theparent metal surface. The applied coating or layer of dopant is thinrelative to the thickness of the parent metal body, and growth orformation of the oxidation reaction product into the permeable bed orpreform extends to substantially beyond the dopant layer, i.e., tobeyond the depth of the applied dopant layer. Such layer of dopantmaterial may be applied by painting, dipping, silk screening,evaporating, or otherwise applying the dopant material in liquid orpaste form, or by sputtering, or by simply depositing a layer of a solidparticulate dopant or a solid thin sheet or film of dopant onto thesurface of the parent metal. The dopant material may, but need not,include either organic or inorganic binders, vehicles, solvents, and/orthickeners. More preferably, the dopant materials are applied as powdersto the surface of the parent metal or dispersed through at least aportion of the filler. One particularly preferred method of applying thedopants to the parent metal surface is to utilize a liquid suspension ofthe dopants in a water/organic binder mixture sprayed onto a parentmetal surface in order to obtain an adherent coating which facilitateshandling of the doped parent metal prior to processing.

The dopant materials when used externally are usually applied to aportion of a surface of the parent metal as a uniform coating thereon.The quantity of dopant is effective over a wide range relative to theamount of parent metal to which it is applied and, in the case ofaluminum, experiments have failed to identify either upper or loweroperable limits. For example, when utilizing silicon in the form ofsilicon dioxide externally applied as the dopant for an aluminum-basedparent metal using air or oxygen as the oxidant, quantities as low as0.00003 gram of silicon per gram of parent metal, or about 0.0001 gramof silicon per square centimeter of exposed parent metal surface,together with a second dopant having a source of magnesium and/or zincproduce the polycrystalline ceramic growth phenomenon. It also has beenfound that a ceramic structure is achievable from an aluminum parentmetal containing silicon using air or oxygen as the oxidant by using MgOas the dopant in an amount grater than about 0.0008 gram of MG per gramof parent metal to be oxidized and greater than 0.003 gram of Mg persquare centimeter of parent metal surface upon which the MgO is applied.It appears that to some degree an increase in the quantity of dopantmaterials may decrease the reaction time necessary to produce theceramic composite, but this will depend upon such factors as type ofdopant, the parent metal and the reaction conditions.

Where the parent metal is aluminum internally doped with magnesium andthe oxidizing medium ia air or oxygen, it has been observed thatmagnesium is at least partially oxidized out of the alloy attemperatures of from about 820° to 950° C. In such instances ofmagnesium-doped systems, the magnesium forms a magnesium oxide and/ormagnesium aluminate spinel phase at the surface of the molten aluminumalloy and during the growth process such magnesium compounds remainprimarily at the initial oxide surface of the parent metal alloy (i.e.,the "initiation surface") in the growing ceramic structure. Thus, insuch magnesium-doped systems, an aluminum oxide-based structure isproduced apart from the relatively thin layer of magnesium aluminatespinel at the initiation surface. Where desired, this initiation surfacecan be readily removed as by grinding, machining, polishing or gritblasting.

The following examples are provided to illustrate the method and resultsof this invention.

EXAMPLE 1

Filler materials for the grown ceramic compositions of this inventionwere fabricated by crushing and milling of ceramic bodies preparedaccording to the methods of the Commonly Owned Patents and PatentApplications. Specifically, bars of a commercial aluminum alloy (aslightly impure version of alloy 380.1, described further below) wereconverted to ceramic by oxidizing in air at 1080° C. for 72 hours,sufficient time to complete the reaction of the aluminum parent metal.During this process the bars were supported in beds of aluminum oxideparticles (Norton E-1 Alundum, 90 mesh particle size) and oxidationoccurred from the exposed surface of the metal toward the airatmosphere. After cooling to ambient temperature, the grown ceramicpieces were separated from any loosely adhering particles of thebedding, from the thin oxide skin which had grown on the non-exposedmetal surfaces, and from any residual metal remaining in the bedding.

These grown ceramic pieces were converted to particles for use as acomposite filler by a combination of crushing and milling. Specifically,the materials were first crushed to a maximum particle size of 1/4 inchby jaw crushing and then reduced further by dry vibratory milling for 24hours. The resulting powder was sieved to separate out the -100/+200mesh size fraction for the composite filler application.

As a control or comparison material, fused alumina particles (Norton 38Alundum) originally of 14 mesh particle size were crushed in a rollcrusher, dry ball milled, and sieved to separate out the -100/+200 meshfraction, i.e., the same mesh fraction as was selected for the grown andcrushed filler material.

Ceramic composite bodies were prepared using the two different fillersfor comparison purposes. Two high alumina refractory boats wereinitially filled to a depth of approximately 1/2 inch with a level layerof wollastonite, a material which acts as a barrier to the oxidationreaction process. A 9"×2"×1/2" bar of aluminum alloy 380.1 was placed ontop of the wollastonite layer in each boat. This alloy contains, inaddition to aluminum, nominally (by weight) about 7.5-9.5% silicon,3.0-4.0% copper, <2.9%; zinc, <1.0% iron, <0.5% manganese, <0.5% nickel,<0.35% tin, and less than 0.1% magnesium, however other samples of the380.1 alloy lot used in this work were found to contain approximately0.17-0.18% magnesium, a potentially important deviation from the nominalspecification since magnesium is an established dopant or promoter ofthe oxidation reaction. The alloy bars were then surrounded on all sidesbut the bottom with particles of the filler materials to a depth of atleast about 1/2 inch, with one boat using the grown and crushed fillerand the other boat using the fused alumina filler.

The refractory boats, filled as described above, were placed in an airfurnace and heated to a temperature of 1000° C. using a firing cycleinvolving a 5-hour ramp to temperature, a 60-hour hold at temperature,and a 5-hour cooling period in the furnace. Subsequently, the grownceramic composite was separated from the barrier and remaining beddingmaterials, and any loosely adhering particles were removed by a lightgrit blasting.

Analysis of the weight gain data for the two samples, taken as thechange in weight of the refractory boat and its contents divided by theinitial weight of aluminum alloy, indicates that approximately the sameamount of reaction occurred into each of the fillers. Specifically, theoxygen pickup was 59% for the case of the grown and crushed filler and56% for the fused alumina filler. However, as shown by a comparison ofFIGS. 1A and 1B, growth into the grown and crushed filler wassignificantly more uniform, which is an important processing advantage.

A comparison of the mechanical properties obtained on specimens cut fromthe two different materials also reveals significant differences assummarized in Table 1. In this table, the modulus of elasticity wasdetermined by a sonic velocity method, the fracture toughness wasmeasured in a conventional Chevron notch test, and the modulus ofrupture was determined in four point bending. The data in the table showa clear superiority in mechanical properties for the material preparedby growing into the grown and crushed filler.

                  TABLE 1                                                         ______________________________________                                        Comparison of Properties                                                                      Filler Material                                                                 Grown and Fused                                             Property          Crushed   Alumina                                           ______________________________________                                        Hardness          84        71                                                (Rockwell A Scale)                                                            Modulus of Elasticity                                                                           316       202                                               (GPa)                                                                         Fracture Toughness                                                                              4.67      2.74                                              (MPa-m.sup.1/2)                                                               Modulus of Rupture                                                                              256       67                                                (MPa)                                                                         ______________________________________                                    

EXAMPLE 2

The procedure of Example 1 was repeated exactly as disclosed therein,except that growth of the final ceramic composite bodies was carried outusing 99.7% pure aluminum as the parent metal rather than the 380.1alloy described in Example 1. In this case growth occurred readily intothe grown and crushed filler material, yielding a weight gain (measuredas in Example 1) of 65% and a quite uniform growth morphology, as shownin FIG. 2A. By contrast, no growth occurred into the fused aluminafiller and, for this specimen shown in FIG. 2B, the weight gain wasnegative, presumably reflecting the elimination of minor amounts ofvolatile constituents from the boat and bedding materials. Thus, in thisExample, growth of the ceramic matrix into the grown and crushed fillerbed was obviously favored over growth into conventional fused aluminaparticles. Mechanical properties of the composite obtained by growthinto the grown and crushed filler material were very similar to, orslightly higher than, those obtained on the material with the samefiller produced as described in Example 1.

What is claimed is:
 1. A self-supporting ceramic composite bodycomprising:a mass of filler material comprising (i) a second particulateceramic composite filler material which comprises a first fillermaterial embedded by a first ceramic matrix comprising a first oxidationreaction product of a first parent metal and a first oxidant and (ii) atleast a third filler material; and a three-dimensionally interconnectedsecond ceramic matrix comprising a second oxidation reaction product ofa second parent metal and a second oxidant which embeds said mass offiller material.
 2. A self-supporting ceramic composite bodycomprising:a second particulate ceramic composite filler material whichcomprises a first filler material embedded by a first ceramic matrixcomprising a first oxidation reaction product of a first parent metaland a first oxidant, wherein said first filler material has a differentcomposition than said first oxidation reaction product; and athree-dimensionally interconnected second ceramic matrix comprising asecond oxidation reaction product of a second parent metal and a secondoxidant which embeds said second particulate ceramic composite fillermaterial.
 3. A self-supporting ceramic composite body comprising:apreform of a second particulate ceramic composite filler material whichcomprises a first filler material embedded by a first ceramic matrixcomprising a first oxidation reaction product of a first parent metaland first oxidant; and a three-dimensionally interconnected secondceramic matrix comprising a second oxidation reaction product of asecond parent metal and a second oxidant which embeds said preform ofsecond particulate ceramic composite filler material.
 4. Theself-supporting ceramic composite body of claim 1, wherein said secondceramic matrix comprises at least one of unreacted parent metal andvoids.
 5. The self-supporting ceramic composite body of claim 2, whereinsaid second ceramic matrix comprises at least one of unreacted parentmetal and voids.
 6. The self-supporting ceramic composite body of claim3, wherein said second ceramic matrix comprises at least one orunreacted parent metal and voids.
 7. The self-supporting ceramiccomposite body of claim 1, wherein said mass of filler material ispresent in a layered form so that the volume percent of said at leastone additional filler material is greater in at least one portion ofsaid mass of filler material than in another portion of said mass offiller material.
 8. The self-supporting ceramic composite body of claim4, wherein said second ceramic matrix comprises voids and furtherwherein the volume percent of said voids is greater in at least oneportion of said second ceramic matrix than in another portion of saidsecond ceramic matrix.
 9. The self-supporting ceramic composite body ofclaim 5, wherein said second ceramic matrix comprises voids and furtherwherein the volume percent of said voids is greater in at least oneportion of said second ceramic matrix than in another portion of saidsecond ceramic matrix.
 10. The self-supporting ceramic composite body ofclaim 6, wherein said second ceramic matrix comprises voids and furtherwherein the volume percent of said voids is greater in at least oneportion of said second ceramic matrix than in another portion of saidsecond ceramic matrix.
 11. The self-supporting ceramic composite body ofclaim 1, wherein said first oxidation reaction product and said secondoxidation reaction product have essentially the same composition andfurther wherein said first oxidation reaction product and said secondoxidation reaction product comprise at least one material selected fromthe group consisting of an oxide, nitride, boride or carbide of at leastone parent metal selected from the group consisting of aluminum,silicon, titanium, hafnium and zirconium.
 12. The self-supportingceramic composite body of claim 2, wherein said first oxidation reactionproduct and said second oxidation reaction product have essentially thesame composition and further wherein said first oxidation reactionproduct and said second oxidation reaction product comprise at least onematerial selected from the group consisting of an oxide, nitride, borideor carbide of at least one parent metal selected from the groupconsisting of aluminum, silicon, titanium, hafnium and zirconium. 13.The self-supporting ceramic composite body of claim 3, wherein saidfirst oxidation reaction product and said second oxidation reactionproduct have essentially the same composition and further wherein saidfirst oxidation reaction product and said second oxidation reactionproduct comprise at least one material selected from the groupconsisting of an oxide, nitride, boride or carbide of at least oneparent metal selected from the group consisting of aluminum, silicon,titanium, hafnium and zirconium.
 14. The self-supporting ceramiccomposite body of claim 1, wherein said first oxidation reaction productand said second oxidation reaction product are different in compositionand further wherein said first oxidation reaction product and saidsecond oxidation reaction product comprise at least one materialselected from the group consisting of an oxide, nitride, boride orcarbide of at least one parent metal selected from the group consistingof aluminum, silicon, titanium, hafnium and zirconium.
 15. Theself-supporting ceramic composite body of claim 2, wherein said firstoxidation reaction product and said second oxidation reaction productare different in composition and further wherein said first oxidationreaction product and said second oxidation reaction product comprise atleast one material selected from the group consisting of an oxide,nitride, boride or carbide of at least one parent metal selected fromthe group consisting of aluminum, silicon, titanium, hafnium andzirconium.
 16. The self-supporting ceramic composite body of claim 3,wherein said first oxidation reaction product and said second oxidationreaction product are different in composition and further wherein saidfirst oxidation reaction product and said second oxidation reactionproduct comprise at least one material selected from the groupconsisting of an oxide, nitride, boride or carbide of at least oneparent metal selected from the group consisting of aluminum, silicon,titanium, hafnium and zirconium.
 17. The self-supporting ceramiccomposite body of claim 1, wherein said additional filler materialcomprises at least one material selected from the group consisting ofalumina, magnesia, hafnia, zirconia, silicon carbide, silicon nitride,zirconium nitride and titanium nitride.
 18. The self-supporting ceramiccomposite body of claim 1, wherein said first oxidation reaction productcomprises alumina, said first filler material comprises alumina, saidadditional filler material comprises alumina and said second oxidationreaction product comprises alumina.
 19. The self-supporting ceramiccomposite body of claim 2, wherein said first oxidation reaction productcomprises alumina, said first filler material comprises silicon carbideand said second oxidation reaction product comprises alumina.